Preschooler with left hip pain

Radiograph and ultrasound of septic arthritis of the hip
AP radiograph of the pelvis was unremarkable. Sagittal US of the hips shows a large anechoic fluid collection in the anterior recess of the left femur, which is not present in the right hip. This left hip effusion, when tapped under ultrasound guidance, returned green pus.

The diagnosis was septic arthritis of hip.